“Okay, how about I take a stab at pointing out just a couple of the problems with the concept of time travel myself? It is almost trivially easy to “prove” time travel cannot exist. Let’s try a simple thought experiment:

Imagine you are a researcher who has come up with a concept for a time machine and after much labor, you’re just about finished with your first test prototype. This prototype is only large enough to send something about the size of a shoe back in time precisely one day. You decide that the first test subject should use some inanimate object like an old shoe. Tomorrow the machine should be ready for its initial test run. To prepare, you go to your closet, find an old shoe and set it next to the machine. Suddenly an identical shoe slides out of the back of the yet unfinished device! Absolute proof that it’s going to work! You excitedly continue work on the final details.

The grand day finally arrived; your time machine is ready to go. You look down at the two identical shoes sitting there and realize that the initial test has to be successful since your shoe already appeared from the machine yesterday. Why, you think to yourself, do I need to send the old shoe through as my first test since I already know that test was successful. So, you decide instead to skip ahead to the second test – sending a live subject through. You take a lab rat from its cage and drop it into the machine.

Hold on, what just happened? Let’s think about this for a moment. You just substituted the rat for the shoe! Where did the shoe that appeared yesterday come from and where did the rat just disappear to? Did you just vaporize the rat? Did the shoe simply appear out of nothing?? Did the rat turn into a shoe??? These are causality paradoxes that obviously can never happen! (So far, this is really just a variation of the infamous “grandfather paradox”.)

Now imagine that you realize these paradoxical questions shortly before dropping the rat into the machine. You hesitate then put the rat back feeling that you MUST put the shoe through to avoid these irrational problems. There is still the problem of a shoe appearing yesterday out of nothingness but you think perhaps the Universe can handle a temporary anomaly such as this so long as it’s paid back in due time, in this case by dropping the shoe into the machine and erasing it from the future timeline. Yes, there seemed to exist two copies of the shoe – but only for one day.

But now you’re not sure which shoe is which! If you drop the wrong shoe into the machine, you’re created yet another paradox. The shoe that appeared yesterday could be the shoe that you put into the machine today. In this case, where and when was that shoe created? It would have popped into existence out of nothing for one day and then simply vanished from the Universe. Another interpretation is that the shoe is stuck in an infinite time loop – appearing out of the machine, being dropped into the machine, going back to appear out of the machine, etc. forever. Just one more paradox without a rational solution! You hesitate again wondering what you should do. You ponder the conundrum until you realize that it too late. It’s now more than a day since the shoe came tumbling out of your time machine. You stand there with a shoe that apparently just came into being out of nothing…

In this scenario the Universe just gained mass in the form of an old shoe. To our understanding, this cannot happen! We know there is a set amount of mass/energy in the Universe that came out of the Big Bang. That’s all we get – no more, no less. (Keep in mind that mass and energy are equated through the handy expression e=mc^2.) A shoe could only appear at the cost of some other mass/energy. Here, the shoe simply appeared. This just can’t happen; there really is no such thing as free lunch in this Universe. (And just in case you missed it, that was a clue.)”